The present invention relates to a coated cutting tool (cemented carbide insert) particularly useful for finishing operations in milling of grey cast iron.
Grey cast iron is a material which, in general, is reasonably easy to machine with cemented carbide tools and long tool life can often be obtained. However, the machinability of cast iron can vary considerably. The tool life may be influenced significantly by small variations in the chemical composition within the material. These variations may be related to the casting technique used such as the cooling conditions. In the finishing operation, the surface finish of the machined work piece is extremely important. It is also as important to avoid frittering of the work piece on the exit side. This puts very high demands on the wear behavior of the cutting edge of the cutting tool, especially close to the corner between the main cutting edge and the parallel land which creates the surface of the work piece.
Measures can be taken to improve the cutting performance with respect to a specific property, but often such action may have a negative influence on other cutting properties as indicated below:
a very sharp cutting edge, i.e., with an edge radius (ER) of about 15 .mu.m or less, minimizes the risk of frittering of the work piece and normally gives a very good surface finish on the component. However, this makes the cutting edge very sensitive to microchipping and for coated cutting inserts also sensitive to flaking; PA1 uncoated cutting inserts with high toughness (e.g., a cemented carbide with a high binder phase content) can be used with a very sharp cutting edge but they have a poor wear resistance and a short tool life; PA1 coating of the inserts in order to improve the wear resistance properties are commonly used. But inserts with very sharp cutting edges are particularly susceptible to decarburization and eta phase formation when subjected to a conventional CVD coating process. The eta phase embrittles the edge line and decreases the flaking resistance and thus also the tool life due to bad surface finish of the work piece. Therefore, conventionally CVD coated inserts generally have an edge radius of about 25 .mu.m as a minimum; and PA1 an inner layer of TiC.sub.x N.sub.y with x+y=1 with a thickness of 0.1-2 .mu.m; PA1 a layer of TiC.sub.x N.sub.y where x+y=1 having a thickness of 0.5-3 .mu.m; and PA1 an outer layer of TiC.sub.x N.sub.y with x+y=1 with a thickness of 0.5-2.5 .mu.m.
PVD coating, using known technique, is recognized to make possible the coating of very sharp edges without eta phase formation. However, in the milling of grey cast iron, severe flaking of this type of coating often occurs which deteriorates the wear resistance and tool life of the insert.
The normally used process parameters in conventional CVD technique, e.g., temperature, pressure and gas composition are a compromise in order to obtain the best combination of growth rate, uniformity and cutting properties avoiding, e.g., undesirable reactions between gas phase and the cemented carbide body, such as etching of the binder phase or decarburization of the insert surface especially the edge line.
A method to avoid decarburization using intermediate oxygen-containing layers is described in certain prior art instances, e.g., U.S. Pat. No. 5,135,801.